I visited a class by Kanetsuka Sensei where he had students non stop cutting up down up down using big tuck tyres to hit. Thus he was 'teaching' them to get rid of their shoulders and to experience the 'bounce back' feeling at the same time. Quite creative.
This tanren practice has a history in Iwama: http://www.youtube.com/watch?v=isAYqzkBTsk&t=15m50s
Latter, Saito sensei used a car tire instead.